ANDROID: iommu/arm-smmu-v3-kvm: Fix missing atomic_mc init

init_hyp_memcache() ensures kvm_hyp_memcache::mapping will stay
unallocated. This member is only useful for CPU stage-2 page-table
pages.

In the case where atomic_pages is 0, an uninitialized atomic_mc will
lead to HVC with broken arguments.

Bug: 357781595
Bug: 348382247
Change-Id: Icb9399a3db86a5412189acb78d91a1b64b988ae8
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>